1. Field
The present disclosure relates generally to testing structures and, in particular, to testing a structure using lamb waves propagating through the structure. Still more particularly, the present disclosure relates to a method and apparatus for identifying lamb wave modes in lamb waves propagating through a structure.
2. Background
Non-destructive inspection of structures for aircraft is performed. The non-destructive inspection is performed to determine whether inconsistencies or changes have occurred within the structures. These structures may be inspected before being assembled or associated with the aircraft during manufacturing of an aircraft. These aircraft are tested to determine whether they meet design parameters for the parts.
Additionally, tests also may be performed on the structures after the structures have been assembled to form the aircraft. This testing may be performed during maintenance or during use of the aircraft.
The inspections may be performed by testing the structure to determine whether a particular structure may need reworking or replacement. This inspection may be performed by setting up equipment in an area to test the structure. The testing also may be performed using health monitoring systems integrated into the structure and/or the aircraft.
The inspection may involve various types of systems, such as ultrasonic systems and x-ray systems. With x-ray systems, x-rays are sent through a structure that is to be tested. Images generated by this transmission of x-rays may be examined to determine whether inconsistencies or changes in the structure have occurred.
With ultrasonic systems, transducers are used to generate signals in the structure. These signals are detected and analyzed to identify the health of the structure. These signals propagating the structure may take the form of lamb waves.
Lamb waves are waves that propagate through a structure. In these examples, lamb waves propagate through a solid structure. Lamb waves are waves with a particle motion that lies in a plane in the structure and the direction of wave propagation. The wave propagation of these types of waves is guided by the boundary of the structure in which they propagate. With lamb waves, these waves have a number of different modes. The modes may include symmetric zero order modes and anti-symmetric zero order modes. These modes exist over larger frequency spectrums, as compared to other types of modes that may be present.
In testing structures, lamb waves may include a number of modes that may be undesirable or unneeded for determining the health of a structure. These other modes may increase the difficulty of interpreting the health of the structure.
These signals detected by the testing system may be interpreted to determine the health of the structure. For example, the data gathered from the tests may be compared to a baseline or a history of data from which the structure was in a condition considered suitable for use. If the test data and the baseline are the same or close enough to each other, the structure may be considered to be healthy. Otherwise, reworking or replacement of the structure may occur. In other test systems, the test data may be used with a model of the structure in a desired condition. This comparison may be used to identify the health of the structure.
Accordingly, it would be advantageous to have a method and apparatus which takes into account one or more of the issues discussed above, as well as possibly other issues.